This invention relates to a method for the preparation of superconducting compositions. More particularly, the present invention relates to a method for the preparation of high temperature superconducting oxides using solution chemistry.
Since the recent discovery of superconductivity in the La-Ba-Cu-O system, several new crystalline oxide phases of the general formula MBa.sub.2 Cu.sub.3 O.sub.7 -y wherein M represents yttrium and the rare earth elements and y is an integer ranging from 0.1 to 0.8 have been discovered. These compositions evidence superconducting transition temperatures, T.sub.c, as high as 95K and extrapolated critical fields of the order of 300 Teslas at 4.2.degree.K. Accordingly, these discoveries have generated significant technological interest and have sparked intense studies of processing techniques for preparing these materials in bulk or thin film form. Heretofore, superconducting thin films have been prepared by molecular beam epitaxy, evaporation, sputtering and pulsed laser evaporation, the properties of the resultant compositions being similar to that of the bulk material.
The new high Tc oxides are typically obtained by conventional ceramic techniques which involve mixing oxides with the appropriate carbonates, oxalates or nitrates and firing the resultant mixture. This reaction proceeds by diffusion in the solid state and hence requires temperatures ranging up to 950.degree. C. with reaction times of at least 48 hours in order to obtain well crystallized materials. Unfortunately, a major difficulty encountered in preparing the 95K superconducting compositions by this method is the imperfect homogeneity of the resultant composition, so necessitating regrinding, repressing and/or refiring to yield the homogeneity required for device applications. It is this heterogeneity of the ceramic materials and uneconomical processing which contribute to poor superconducting characteristics and have prompted the search for new methodologies.